Japanese Patent Laid-open Nos. H2(1990)-139893, H2(1990)-196475 and H2(1990)-196475 disclose an organic EL element provided with an inorganic semiconductive thin layer and an organic luminescent layer between electrodes for easier injection of electrons and the like. In the organic EL element, an inorganic semiconductive thin layer is formed on an anode, and an organic luminescent layer and a cathode are further formed thereon, which inorganic semiconductive thin layer is made of an inorganic semiconductive material such as carbon, germanium, silicon, tin, silicon carbide, boron nitride, boron phosphide and gallium nitride.
Japanese Patent Laid-open Nos. H10(1998)-88120 and 2000-150161 disclose an organic EL element being of structure, positive hole injecting layer/luminescent layer/electron-injecting layer for improving its luminance (luminescent brightness). Amine materials capable of transporting positively charged holes (hereinafter referred to as positive holes) are used as a luminescent material, and tris(8-hydroxyquinolinate)aluminum (Alq), bis(2-methyl-8-hydroxyquinolinate)(p-cyanophenolate)gallium and the like are used in the electron-injecting layer.
However, in the organic EL elements disclosed by H2(1990)-139893 and H2(1990)-196475, the mobility of electrons injected from the cathode is relatively reduced by the inorganic semiconductive thin layer, resulting in a decrease in luminous efficiency. On other words, although electrons are desired to recombine with positive holes around the center of the organic luminescent layer for efficient luminescence, they may re-couple around the inorganic semiconductive thin layer, thereby losing luminescence or reducing the ratio of re-coupling. Consequently, the luminescence in the organic luminescent layer is decreased.
In the organic EL elements disclosed by H10(1998)-88120 and 2000-150161, the materials capable of injecting electrons such as Alq, which are used in the electron injecting layer, are liable to be degraded with shorter half-lives.
The inventors have made efforts to solve the above problems and then found that an organic EL element can be obtained of which luminance is higher even at low voltages, e.g., direct current 10V, and half-life is remarkably lengthened by certain structures. According to a first structure, an inorganic compound layer is provided between an organic luminescent layer and a cathode layer, and certain aromatic amine compounds are used in the organic luminescent layer. According to a second structure, a layer containing a reductive dopant (hereinafter referred to as a reductive dopant layer) is provided between an organic luminescent layer and a cathode layer, and certain aromatic amine compounds are used in the organic luminescent layer. According to a third structure, a certain electron injecting layer is provided between an organic luminescent layer and an anode, and certain aromatic amine compounds are used in the organic luminescent layer.
An object of the present invention is therefore to provide an organic EL element with high luminance even at low driving voltages, and with remarkable long half-life.